Like bevills1 said, you should be able to access your HDD fine if you have your Master & Slave drives set correctly. Windows should show your drive when you boot back up on your 80GB HDD. If it shows it as unformatted then most likely your drive has corruption/damaged index/damaged tables/damaged partitions. The only way to fix the problem is:
1. Fix the damaged index/table/partitions
2. Retrieve the data with software
3. Reformat the HDD (you would lose your data)
Otherwise your HDD is damaged beyond repairs. I always keep an external usb/firewire case around because sometimes, you never know when it will help you recover your files. You could possibly recover your HDD files with an external case or you can try software.
Get Data Back NTFS/FAT Free trial Of course software is not free but sometimes it works to recover your files. I have recovered one of my friend's data files with this software before due to damaged indexes and partitions. And like Steve said, make sure you have Windows XP Service Pack 2 installed, SP2 has support for breaking the 137GB barrier on HDD. Otherwise your drive may show up as unformatted.